Females of certain fruit fly species mate with multiple males, which gives the males motivation to mate for as long as they possibly can. But in species where the female chooses only one mate for life, males are still jealously protective...but why?

That's the question pondered by researchers at the University of Liverpool. Dr. Anne Lize of the Institute of Integrative Biology explains:

"We already knew, from previous studies, that male insects evolve physical and behavioural characteristics to make them a stronger reproductive competitor for females that mate with many males. Some butterflies, for example, have evolved disproportionately large testes so that males can deliver increased sperm numbers. Fruitflies have behavioural reactions to the presence of rival males that result in prolonged reproduction with females to ensure fertilisation before the female mates with other males.

"We wanted to understand how this compared to fruitfly species where females only mated once, to see whether drivers of evolutionary change are different in species where sperm competition is low. What we observed might be considered the evolutionary equivalent of male 'paranoia'. Males more than doubled the length of time they mated with a female after they had been in the presence of other males."

So what's driving these males to prolong their mating, even though they should "know" that their female partners will remain monogamous? This fruit fly paranoia might be because, on rare occasions, females do end up mating with a second male, and on some weird level the rarity of this action provokes a disproportionate response from the male.

That may seem rather unlikely - and, honestly, it does - but Dr. Lize suggests the truth isn't too far from that:

"There doesn't appear to be an immediate biological reason as to why they would do this, but it is perhaps to ensure that the female remains fertile with one male's sperm for her lifespan, even though the chance of reproducing with another is remote.

"Our findings are significant because they demonstrate that particular behaviours, such as the response to competitor males, can have diverse evolutionary drivers. This takes us a step closer to understanding the differences between males that have evolved within species where individual females mate with many males and those have just one partner."